Toshiki Kashû

A man named Shouichi Tsugami has lost his memories. He doesn't know who he is, where he came from, or how he came upon his peculiar circumstances. Tsugami, seemingly for no reason, transforms into a powerful superhuman, Agito, whenever in the presence of the beings referred by the police as the "Unknown", a race of powerful monsters that have been causing murders around Tokyo, targeting certain people as their prey. In response, the police department unleash their newest weapon: the G3 powersuit (mydramalist.com)

Shouichi, Ryo, and Hikawa have to combat a new wave of Unknowns, but are faced with another threat in the form of G4, another Battle Suit designed to fight the Unknowns, but is potentially stronger and is on the wrong side.

Nobuhiko Tsukikage, the caretaker of Tsukasa's sister, warns him that Dai Shocker plans to invade the Earth. He suggests that he should battle all of the Riders to see which one is the stronger to battle against the evil organization.